Dolphins are recognised for their natural ability to use sound, or echolocation, to detect the shape, size, and speed of objects under water. But the marine mammal’s visual perception of the world is less well-understood.

In a new study, led by Masaki Tomonaga from Kyoto University in Japan, researchers looked at how bottlenose dolphins perceive a range of simple, two-dimensional objects compared with chimpanzees and humans.

Previous studies of dolphin vision have used complicated, three-dimensional shapes, making it hard for researchers to identify the types of visual “cues” — like curves or diagonal lines — that dolphins use to suss out differences or similarities among objects.

Dolphins have limited colour vision and poorer visual acuity, or clearness of vision from a specific distance, both in air and under water than primates. Despite these differences, bottlenose dolphins “perceive the world in fundamentally similar ways” to other primates, the study authors wrote in the journal Scientific Reports.

The researchers tested the visual perception of three wild-born bottlenose dolphins — Peace, Tino, and Eagle — currently living in the Port of Nahoya Public Aquarium in Japan.

Scientific ReportsStimuli and their related features used in the study. The right area shows the features contained within each stimulus. The study included seven distinct features.

Nine simple geometric shapes with distinguishing features, such open-endedness or right angles, were used in a matching task. The shapes and their various features are shown in the diagram to the right.

In a technique known as “delayed-matching,” a dolphin was trained to touch its nose to a sample shape (a double-concave, for instance) that was shown above the water by a person. The sample shape was then removed and the dolphin was presented with a pair of shapes, one in each each hand (a double-concave and a D-shape). The dolphin was required to choose the shape that was identical to the sample shape.

By combining nine shapes into pairs, 36 pairings were included in the study.

Comparable matching tests, using pictures of the 36 pairs, were done with chimpanzees and humans. Seven chimpanzees were presented on a computer screen with colour photographs of the same shapes shown to the dolphins. When given two choices, the chimpanzees had to touch the shape that was identical to the sample. In the human experiment, 20 volunteers used a rating scale to judge the similarity of each pair, which were printed on a single sheet of paper.

The results of the tests for each species is below. All three dolphins and the chimpanzees displayed similar patterns of confusion — they had a tougher time discriminating between shapes with the same features (such as a circle and D-shape or a U-shape and H-shape), meaning they view these shapes as “similar.”

Considering dolphins and primates live in very different environments, their visual abilities were surprisingly similar, the researchers write.

But there was one distinction — all three species relied on different details to determine “similarity.”

“The weight given to each feature in determining perceptual similarity differed slightly among species,” the researchers wrote.

For instance, humans perceived curved-shapes as more similar than dolphins did, but perceived shapes featuring acute angles as less similar than both dolphins and chimpanzees. Chimpanzees, on the hand, perceived open-ended shapes as more similar than the other two species did.

It’s still not clear why these differences emerged, the researchers said in the study, but point to some procedural inconsistencies. Humans, for example, were shown shapes that were much smaller than the shapes presented to the dolphins and chimpanzees. It’s possible that the reduced size caused the human group to pay more attention to features like closure, the authors said.

It’s also known that humans first notice the whole shape, whereas other animals pay more attention to finer details, Tomonaga told Business Insider over email.